Triggers
Trigger nodes start a flow. Each flow has exactly one, fixed by the flow's kind.
You don't add trigger nodes from the node picker. The kind of a flow decides its trigger, and the trigger node is always the first node on the canvas. Each trigger has one way out, out.
Slash command
Runs when someone uses a command.
Right-click command
Runs from the Apps menu of a member or a message.
Event
Runs when something happens in a server.
Schedule
Runs on a timer.
Button action
Runs when a button or menu is used.
Sub-flow
Runs when another flow calls it.
Webhook
Runs when an outside service calls its address.